Communication devices, including digital cameras and high-definition digital camcorders are ubiquitously used and require an increasingly higher quality of service.
There is a growing need for reliable communication devices with high recording capacities that are user friendly and offer high image quality.
When images such as video and photographs are viewed on a display device including a HD (high-definition) television, the required bit rates for the transmission of data between the imaging device and the display device are in the range of several gigabits per second (Gbps).
Similar bit rates are necessary for the transmission of data between an imaging device and a storage device or physical carrier dedicated to the storage of multimedia data (audio and video data).
To prevent loss of quality during the transfer of images, a digital wire link such as an HDMI (high-definition multimedia interface) cable is at least necessary.
Indeed high-definition non-compressed multimedia data are transmitted in raw mode, it being understood that almost no processing and no compression is performed.
Raw data as recorded by the sensor of the imaging device can therefore be rendered without loss of quality.
Moreover, in home communication, raw data needs also to be transmitted almost in real time.
However, the use of a wired link in home communications systems has several drawbacks.
For example, a wired link between a camera and a television set has several limitations.
On the television set side, the connection systems may be difficult to access or may even not be available.
On the camera side, the connection systems are very small in size and may be concealed by covers, thereby making it difficult to connect the cable. In addition, it can be very difficult to move the camera or the screen when all devices are connected.
Similarly, in case cables are integrated in the walls of the house it is impossible to modify the installation. One approach for overcoming these drawbacks is the use of wireless connections between the communication devices.
However, said systems need to support data bit rates to the order of several Gigabits per second (Gbps). WiFi systems are operating in the 2.4 GHz and 5 GHz radio bands (as stipulated by the 802.11.a/b/g/n standard) and are not suited to reach the target bit rates. It is therefore necessary to use communications systems in a radio band of higher frequencies. The radio band around 60 GHz is a suitable candidate. When using an extensive bandwidth, 60 GHz radio communications systems are particularly well suited to transmit data at very high bit rates. In order to obtain high quality radio communications (i.e. low error bit rate) and sufficient radio range between two communication devices without having to transmit at unauthorized power levels, it is necessary to use directional (or selective) antennas enabling line of sight (LOS) transmission. Consequently, narrow beam forming techniques are necessary for wireless transmission with high throughput bit rate.
During the discovery phase, each pair of nodes of the wireless network has to initiate the communication parameters. It is therefore necessary to configure the antenna angle in order to obtain the best quality with the radio frequency (RF) link.
Communication parameters can be transmitted with a low bit rate and therefore allow decreasing needs in the budget of the RF link (e.g. antenna gain). This in turn allows a wide antenna beam to be formed in order to detect all the nodes within reach.
Consequently, the antenna has to form both a narrow and a wide beam during subsequent phases.
The antenna needed in the above-mentioned applications shall therefore be reconfigurable so as to obtain a narrow beam in azimuth, while having a large beam in elevation.
More specifically, the antenna required in such circumstances needs, by way of example, to satisfy the following requirements:
bandwidth: 57 to 64 GHz;
azimuth pattern: <15 degrees;
elevation pattern: >70 degrees;
azimuth pattern coverage (beam directivity): −70 to +70 degrees.
The problems described above, mainly refer to the setting up of very high bit-rate point-to-point wireless communications between a digital camera (DVC) and an HD television set. It is clear however that the problems may be extended to any context in which it is sought to set up wireless communications between a sender device being an imaging device and a receiver device being a device for data display or data storage.
The so-called smart antennas or reconfigurable antennas are used to reach the distances required by audio and video applications. A smart antenna mainly comprises a network (e.g. an array) of radiating elements distributed on a support. Each radiating element is electronically controlled in phase and power (or gain) in order to form a narrow beam or set of beams in sending and reception mode. Each beam can be steered and controlled. Consequently, this requires a dedicated phase controller and a power amplifier for each antenna element which increases the cost of the antenna.
In order to obtain a narrow beam, several antenna elements have to be powered, which may therefore result in significant consumption of energy. Power consumption is a serious handicap, especially for battery-powered portable devices.
In addition, the geometrical dimensions of the smart antenna are also a strong limitation to small portable devices.
The smart antennas known in the prior art comprise a network of radiating elements (for example 16) laid out in a square array on a substrate. The radiating elements have each a dimension of half the wavelength (i.e. 2.5 mm in case of 60 GHz range) and the space between the antennas elements has to be at least of one quarter of the wavelength. Consequently, the surface of a smart antenna is rather large, which is not very convenient for being integrated in portable devices. This leads to high costs, particularly when the materials used in the manufacture of the antenna comprise a substrate based on semiconductor technology. In the latter case, the final costs for mass market production of portable devices may be too high.
A planar steerable antenna using PCB patch is proposed by Sibeam (product SB9220/SB9210). This antenna sends energy in a large set of predefined directions. The number of possible directions is a function of the number of radiating elements.
However, many radiating elements are needed for such a design. Mutual inductance between the antenna elements is an important drawback for this technique and results in waste of energy through coupling. Also, the inherent symmetry causes energy to be sent in non desired directions. Another drawback is the necessity to adapt both the amplitude and the phase of the signal to be sent to each radiating element. Such an operation is costly at 60 GHz frequency.
In a know manner, spherical electromagnetic lenses are used in steerable antennas. The basic concepts are described by R. Luneburg (Mathematical Theory of Optics, Cambridge University Press, 1964). Spherical lenses are composed of dielectric materials having a gradient of decreasing refractive index. The relative dielectric constant of the lens (commonly referred to as Luneburg lens) follows the following rule:∈r(r)=2−(r/R)2, for r=0, . . . , R; and varies with the radial position r in the lens. Good control of the beam in azimuth is obtained through radiation into the lens of several thin beams along its edges. The Luneburg lens can be used in many applications mainly comprising radar reflectors and high altitude platform receivers. Spherical shapes of the lens are mainly used.
Two implementation techniques of the Luneburg lens are known and consist either in drilling holes as described in S. Rondineau, M. Himdi, J. Sorieux, A Sliced Spherical Luneburg Lens, IEEE Antennas Wireless Propagat. Lett., 2 (2003), 163-166, or using variable dielectric materials in different shapes as described in WO 2007/003653.
Available commercial products are mostly alternatives of satellite dishes, being able to emit radiations at a low elevation. However, they are not suitable for applications requiring a constant angle in elevation and beam steering in azimuth.
Furthermore, beam forming and beam steering techniques are described in prior art. In WO2009013248, an antenna system is considered based on a lens being able to configure either a narrow beam or a sector-shaped (or wide) beam. The antenna system has a radiation diagram that can be reconfigured. This antenna is well adapted for the automotive radar application, but presents limitations for a wireless portable device. Their use in portable devices is not compatible due to the form and volume taken by the spherical or hemispherical lens. It is also difficult to manufacture said antennas from an industrial point of view. In particular, the assembly of the concentric homogeneous dielectric shells forming a spherical lens or hemispherical lens remains a problem. The number of the antenna sources in a given plane is also a strong limitation, particularly when considering the requirements for the azimuth angle of 160° and 10° for the narrow beam in 16 different directions. This implementation is thus not suitable.
Another solution is proposed in US 2008048921 where the antenna can generate multiple beams.
A current problem, known in the prior art relates to the design of antennas capable of beam forming (directional lobes) both in transmission and reception and concerns the interconnections between the individual radiating elements of the antenna array and the electronic circuit. In section VII of the article entitled: Design of millimetre-wave CMOS radio, IEEE Transaction circuit and system—vol. 56 No 1 January 2009, the authors emphasise the problem of interconnections generating both phase shifts and signal amplitude level shifts, while creating additional losses and spurious couplings that are detrimental to the intrinsic characteristics of the antenna. In addition, it is even more difficult to design feeder circuit routing guaranteeing accuracy during manufacturing.